The present invention relates to a process and die for forming an end cap in a ceramic tube in which the ceramic tube is extruded into an end cap forming cavity defined in the die. More particularly, the present invention relates to such a process and die in which ceramic material is backfilled into the end cap forming cavity to compact the ceramic material so that the ceramic material within the end cap has a substantially uniform density.
The manufacture and operation of high temperature gas separation and fuel cell reactors depends on the availability of large numbers of ceramic oxygen transport membranes. In one configuration, these membranes are configured as arrays of thin-walled tubes in shell-in-tube type reactors. Reactor systems using this configuration rely on arrays of tubes within metal reactor shells. However, it has been found that there are severe problems with maintaining gas-tight seals and tube integrity when arrays of open-ended tubes mounted in metal reactors are-thermally cycled to operating temperatures that are in excess of 1000xc2x0 C.
Due to the high failure rate of such open-ended tubes, the industry has sought to develop closed-end tubes. However, this has not been a simple task because for closed-end tubes to be of commercial value, it is important that the operational and performance characteristics remain the same throughout the length of the tube including the tube end. Such tube ends must, therefore, have a uniform thickness density and strength, in relation to the tube lengths.
Ceramic tubes may be made by molding, casting, extrusion, as well as other methods known to those of skilled in the art. Commercial ceramic tubes are typically made by extrusion to provide tubes that have uniform thickness, density and strength throughout the tube length.
The ceramic material usually comprises a ceramic oxide powder in a binder. The ceramic oxide powder/binder system is typically made into a formable paste, extruded through a die to form a tube in a xe2x80x9cgreenxe2x80x9d state, thermally treated to partially remove the binder to leave a bisque fired body, followed by sintering and densification by high temperature heat treatment. If the tubes are to be closed at one end, tube closure or capping is done prior to preparation of the bisque fired body.
Although various means are known for forming open-ended ceramic tubes, tube closure methods of the prior art have proven unsatisfactory for ceramic tubes. Traditionally, tube closure has been accomplished by plugging or capping.
Plugging requires preparation of the plug in a separate operation from formation of the tube. Due to the fragility of the green body, plugging is typically done manually by moistening the plug, inserting it into an open end and molding the pieces together. Closure of tubes by plugging results in the production of tube ends having varying density and strength. Also due to the necessity to carefully control the jointing, tube closure by plugging does not represent a commercially viable means of production.
With respect to capping, extrusion of a tube requires that material be forced through a extrusion die that has an annulus at the center of which is a mandrel. The difference in diameters of the annulus and mandrel governs the tube wall thickness. To ensure uniform tube wall thickness, the mandrel is located centrally in the annulus typically by an array of suspension lets, oriented in a circular pattern at regular intervals such as at a 90-degree spacing, commonly referred to as a xe2x80x9cspiderxe2x80x9d. During extrusion, the material is split into four sections as it passes over the mandrel and xe2x80x9cspiderxe2x80x9d, and then is reunited as it passes beyond the annulus.
With conventional capping, the material is first passed through the annulus and over the mandrel and xe2x80x9cspiderxe2x80x9d and forced into a capping die. Following formation of the cap, the length of tube is extruded. Caps formed by this method, however, typically exhibit xe2x80x9cghostxe2x80x9d fissures from the 4-way division of material over the extrusion mandrel. The reason for this is the division of material produced discontinuities within the density of the ceramic material forming the end cap. Sintering of such caps do not appear to heal or the green body defects result in failure of the formed tubes at the end cap region.
As will be discussed, the present invention provides a method of capping an extruded tube by formation of an end cap having a uniform density to in turn provide a uniform strength through the tube length and end cap.
The present invention provides a process for forming an end cap in an end of a ceramic tube. In accordance with the process, an end cap forming die is positioned against a extrusion die. The end cap forming die has an end cap forming cavity, a backfill reservoir, and a passageway communicating between the backfill reservoir and the end cap forming cavity. The ceramic tube is extruded so that ceramic material forming the end of the ceramic tube is forced into the end cap forming cavity, through the passageway, and into the backfill reservoir. The ceramic material is then forced from the backfill reservoir back through the passageway and into the end cap forming cavity to compact the ceramic material within the end cap forming cavity so that the ceramic material forming the end cap has a substantially uniform density.
A portion of the ceramic material and air can preferably be discharged from the backfill reservoir prior to the ceramic material being forced back to the end cap forming cavity. The extruding of the tube can be suspended prior to the ceramic material being forced from the backfill reservoir.
Preferably, back fill reservoir is elongated and is provided with ports for discharging the portion of the ceramic material and the air. In such embodiment, the ceramic material is forced back into the end cap forming cavity by an elongated plunger projecting into the backfill reservoir. The elongated plunger covers the ports during the forcing of the ceramic material and is retracted so that the ports are uncovered during discharge of the portion of the ceramic material from the backfill reservoir.
In another aspect, the present invention provides an end cap forming die for forming an end cap in an end of a ceramic tube. In accordance with this aspect of the present invention, the die is provided with a body configured to be situated against a extrusion die. The body has an end cap forming cavity to form the end cap, a backfill reservoir to receive ceramic material forming the ceramic tube from the end cap forming cavity, and a passageway communicating between the end cap forming cavity and the backfill reservoir. The end cap forming cavity is positioned so that when the body is situated against the extrusion die, ceramic material forming the end of the ceramic tube is able to be forced into the end cap forming cavity, through the passageway, and into the backfill reservoir. A plunger projects into the backfill reservoir to force the ceramic material from the backfill reservoir back through the passageway and into the end cap forming cavity to compact the ceramic material within the end cap forming cavity so that the ceramic material forming the end cap has a substantially uniform density.
The backfill reservoir can be of elongated configuration and can be provided with ports for discharging the ceramic material and air. The plunger can also be elongated and configured to cover the ports when the ceramic material is forced back into the end cap forming cavity. The plunger retracts to uncover the ports when the ceramic material and air is discharged Preferably, the backfill reservoir, the passageway, and the end cap forming cavity are coaxial.